In many electronic devices, it is desirable to generate a voltage having a magnitude that is greater than a magnitude of a supply voltage providing power to the device. In other applications, it is desirable to generate a polarity that is different from the polarity of the supply voltage providing power to a device. Charge pumps may be used for both of these purposes. Although a wide variety of charge pumps have been developed, many charge pumps use capacitors to obtain a boosted voltage or a voltage having a different polarity.
Typically, a supply voltage is sampled on a first terminal of a capacitor (by charging the capacitor to the supply voltage) during a first phase of a cycle. During a second phase of the cycle, one of the terminals is coupled to a load. If the first terminal of the capacitor is coupled to the load and the second terminal is held at ground, a boosted voltage may be generated. Because the capacitor was charged to the supply voltage during the first phase when the second terminal was connected to ground, the voltage on the first terminal is approximately twice the supply voltage during the second phase. If, during the second phase, the second terminal of the capacitor is coupled to the load and the first terminal is held at ground, a voltage with a reverse polarity may be generated. Because the capacitor was charged to the supply voltage during the first phase when the second terminal was connected to ground, the voltage on the second terminal is approximately a negative supply voltage during the second phase. The charge pump repeatedly alternates between the first and second phases, each cycle generating an output voltage that is approximately twice the supply voltage VAA or of a reversed polarity.
Charge pumps are presently used in a wide variety of applications. For example, charge pumps are typically used in memory devices to provide a negative substrate voltage or to provide a boosted voltage that may be applied to the gate of an NMOS transistor to allow the transistor to couple the supply voltage to an output node. Charge pumps are also used in CMOS imagers to generate voltages of different polarities and magnitudes during various operations carried out by the imagers. For example, charge pumps are commonly used to supply power having a polarity that is different from that of the supply voltage to the imaging array of CMOS imagers.
The time required for a charge pump to output a target voltage is sometimes referred to as a time constant of the charge pump. In general, the time constant of a charge pump driving a resistance load is very short as long as the current demands of the load do not exceed the current that may be supplied by the charge pump. The time constant, however, of a charge pump driving a capacitive load may be very long because the voltage applied to a load incrementally increases through a charge sharing process each cycle. The time constant of the charge pump may affect the magnitude of capacitance relative to the load capacitance, as well as the difference between the supply voltage and the load voltage to which the capacitive load has been charged. The charge pump, thus, may be slow to reach the target voltage because the charge pump may not produce more charge than the combination of the pump capacitance, supply voltage and the load voltage. In addition, charge pumps typically do not compensate for charge lost when the charge pump is inactive.